Automatic transmissions comprise a plurality of hydraulically operated friction devices, such as clutches and brakes, to which hydraulic pressure is selectively applied so as to engage or disengage the friction devices. The friction devices are responsible for executing various functions in the automatic transmission, such as gear ratio selection, gear shifting, and the like.
In an electronic control transmission (ECT), these frictional devices are controlled by a computer system or control unit, e.g., a transmission control unit (TCU), and various sensors and solenoids. ECTs have been known to improve the control of transmission shift points, with less abrupt gear shifting and improved shift smoothness. In addition, the flexibility of the ECT allows for better adaptability to changing conditions.
In a basic ECT design, a plurality of sensors monitor various characteristics of the vehicle, such as the speed of the vehicle, gear position selection, engine speed, throttle position, torque-converter speed/lock, wheel-slip, temperature, and the like. The control unit receives signals from the various sensors and acts on them through one or more fluid control solenoids.
A solenoid is a linear motor with a fixed range of travel. In an ECT, solenoids may be used to operate pistons and valves for accurate control of hydraulic fluid pressure or flow. The solenoids of an ECT system receive an electrical input, normally an electrical current, from a control unit and convert that electrical energy into mechanical energy to open and close a solenoid valve mechanically by providing an amount of output pressure that is a function of the amount of electrical input. The output pressure in turn is used to control the frictional devices.
The relationship between the amount of output pressure and a specific current is established through characterization. This relationship may be inclusive of various errors, e.g., variations in the repeatability of the solenoid, variations in the repeatability of the control unit, friction, corrosion, and the like. Variations or inaccuracies in the output pressure supplied by a solenoid may decrease the shift quality, i.e., may result in abrupt shifting or a decrease in shifting smoothness. It is important to have precise, accurate and repeatable control of the solenoid opening, which in turn allows for precise, accurate and repeatable control of the shift points through the application of precise amounts of hydraulic fluid. Accordingly, it is desirable to provide improved methods of characterizing a solenoid and improved methods for providing more accurate current commands to deliver a precise and accurate pressure output in response to a command current.